The invention concerns two-pulse brushless d.c. motors of the type comprising a wound stator having a least one conductor, each conductor being wound to form at least one concentrated coil, and further having a permanent-magnet rotor. During rotation, the main poles of the rotor induce in each coil an induced voltage formed by positive and negative half-cycles alternating with each other, as a result of each coil being exposed to transitions from north rotor flux to south rotor flux, or vice versa.
In these two-pulse brushless d.c. motors, the voltage induced in each coil exhibits a transitional interval, the number of transitional intervals being equal to the number of main poles of the rotor. During each such transitional interval, the induced voltage in each coil undergoes a transition from one voltage polarity to the opposite voltage polarity. Although such transition from one voltage polarity to the opposite voltage polarity is gradual, i.e., not instantaneous, the voltage polarity change exhibits a slope of relatively high absolute value. Furthermore, during each such transitional interval the voltage induced in each coil contributes to form a total induced voltage the absolute value of which is such as to form an induced voltage valley.
Because the electromagnetically generated torque produced by the motor corresponds somewhat closely to the total induced voltage (total back EMF), any electromagnetic torque generated within the range of rotor positions corresponding to such induced voltage valley will be of relatively low value, especially at the range of rotor positions corresponding to the bottom region of the induced voltage valley. In effect, the electromagnetically generated torque exhibits valleys similar to those exhibited by the total induced voltage. Being thus of low value at such range of rotor angular positions, the electromagnetic torque may be too weak to maintain the rotor in rotation, for example during heavy mechanical loading on the motor shaft, and most especially during motor start-up.
Accordingly, motors of this type are typically provided with some means for furnishing an auxiliary torque, operative during such valleys or gaps in the electromagnetic torque to compensate for the low values of the electromagnetic torque within such valleys. Over the range of rotor positions corresponding to such valleys, or at least corresponding to the bottom regions of such valleys, the auxiliary torque is exerted in a direction the same as the desired rotation direction.
As well known to persons familiar with this type of motor, each commutation of the motor generally occurs at a rotor position corresponding to one of the induced voltage or electromagnetic torque valleys.
Motors of this type have become variously known from the assignee's products and are employed, for example as the drive motor in disk drives. Examples of such motors, and of the driving circuits therefor, can be found in U.S. Pat. Nos. 3,873,897, 3,840,761, 4,099,104 and 4,429,263, the entire disclosures of which are incorporated herein by reference. When these motors are employed in disk drives, they are often provided with a brake, and when the motor is switched on, the brake is disengaged. It will sooner or later happen that the rotor is braked to a stop by the brake in an unlucky position corresponding to one of the aforementioned induced voltage or electromagnetic torque valleys. When the motor is switched on again the only torque available to drive the rotor is aforementioned the auxiliary torque. The auxiliary torque can for example be a mechanically produced auxiliary torque, or a reluctance torque whose shape and functional dependence upon rotor rotation can, in the case of a cylindrical-air-gap motor, be predetermined by the shape of the air gap. This auxiliary torque, then, must turn the rotor into a position in which the electromagnetically generated torque is able to drive the rotor.
With such motors a certain problem can arise from the tolerance variations in the system that controls the time of occurrence of the motor commutation, e.g. tolerance variations in the Hall generators or Hall-IC's that are used for this purpose, or mechanical tolerance variations in the motor. In particular as a result of such tolerance variations the commutation may occur earlier or later than intended. The result of this can be that, at the time of occurrence of the commutation, the electromagnetically generated torque is negative, i.e., seeks to turn the rotor backwards. It is at exactly this time that the auxiliary torque is being relied on to turn the rotor forwards. The sum of the two torques can in such situations be close to zero; i.e., upon disengaging the brake the motor does not start up.